e+e- Pairs from a Nuclear Transition Signaling an Elusive Light Neutral Boson

Electron‐positron pairs have been observed in the 10.95‐MeV 0− → 0+ decay in 16O. The branching ratio of the e+e− pairs compared to the 3.84‐MeV 0− → 2+ γ decay of the level is deduced to be 20(5) × 10−5. This magnetic monopole (M0) transition cannot proceed by γ‐ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral 0− or 1+ boson. Indeed, we do observe a sharp peak in the e+e− angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c2. It may play a role in the current quest for light dark matter in the universe.